As is well known, a semiconductor electronic device, e.g. a power device, comprises a small plate of a semiconductor material known as the “die”, which has a surface area of a few square millimeters and has an electronic circuit integrated monolithically therein. The die has a plurality of terminals, typically in the form of surface contact pads, that are connected electrically to a support which is also a part of the electronic device. This support is known as the “lead frame” and can be provided in different forms, including ceramic substrates, printed circuit boards, pliant circuits, and silicon-based substrates.
The aggregate of the die and the support is encased into a “package” for heat and mechanical protection of the electronic device, both during storage and in use. The package case is typically a plastic material.
Different technologies may be used to connect the die of semiconductor material electrically to the support, of which a commonly used one provides for the use of wire leads and is known as “wire bonding”. In particular, each wire is soldered with one end on a contact pad formed on the die of semiconductor material, and with the other end on a contact pad correspondingly formed on the support to which the die is connected.
The contact pads of a die of semiconductor material are typically made of aluminum or an aluminum alloy, for manufacturing convenience and performance of the device. The wires are usually gold, copper, or alloys thereof, because these materials alloy well with aluminum.
Although semiconductor electronic devices of the above type are advantageous, they have serious drawbacks in that their reliability may be low and their performance may deteriorate rapidly in high-temperature applications. For instance, it is known that power electronic devices are largely used in many automobile vehicles in electronic control units for controlling the operation of engine, drive, and brake systems. These control units are made smaller and smaller in size and include an ever-growing number of functions. This results in increased power density and consequently increased heat generated from the integrated circuits.
With electronic or generic discrete devices, this heat is normally dissipated through their packages, which are designed to also serve this purpose. The heat may additionally or alternately be dissipated using suitable heat sinks.
However, the packages of power electronic devices used in automotive control units, especially those used in engine control units, are not typically able to dissipate the heat properly due to the high ambient temperature (usually above 100° C.) to which the control units and consequently the semiconductor devices therein are exposed. Under this working condition, the performance and reliability of the electronic devices deteriorate rapidly during the device life span, which is therefore not high.
More particularly, it is found that when the above electronic devices are operated at high ambient temperatures (above 140° C.) with electric currents close to their designed maximum, the electrical resistance of the wire-to-pad connection of the die of semiconductor material increases dramatically with time. This behavior has been investigated in more detail, and it has been determined that, in the above conditions, it is caused by a continuous migration of gold atoms from the wire lead into the underlying aluminum layer that forms the contact pad. Therein, the gold atoms “use up” aluminum from the contact pad producing with it a gold/aluminum intermetallic compound that has a high electrical resistance.
It has been further determined that, when a package of an epoxy resin formulated with bromine and antimony-containing fire retardants is used, the above diffusion of gold atoms associates with the formation of voids at the interface between the gold and the aluminum layer thereunder. These voids are responsible for a progressive mechanical deterioration of the soldered joint thereby eventually causing the wire lead to become detached from the contact pad on the die. This behavior is promoted by the bromine and the antimony provided in the composition of the epoxy resin in the package.
Similar problems to those just described also appear, although over a somewhat longer period, when copper wires are used instead of gold ones and/or when the electrical resistance of the wire-to-pad connection of the die of semiconductor material is increased.
Thus, there exists a demand for semiconductor electronic devices, specifically power devices, which should be highly reliable and capable of satisfactory lifelong performance in applications involving high temperature conditions. This demand is more pressing since automotive technology is moving in the direction of installing the control unit directly on, or very close to, the actuating system, that is to say the engine, drive, or brake system. This means that the semiconductor electronic devices of such control units will be required to withstand still higher operating temperatures and, at the same time, also withstand increased shock loads and vibration.